The invention relates to a method of manufacturing a deflection unit for a colour television display tube, of the type having a neck portion containing means for generating a plurality of electron beams and a cone portion with a display screen remote from said neck portion, and further having a pre-adjusted colour purity magnet, said deflection unit comprising line and field deflection coils and a magnetic core which cooperates with said deflection coils.
The invention also relates to a deflection unit manufactured according to the method and to the combination of such a deflection unit with a colour display television tube.
One method is known from the Philips Product Information "Adjustment of Colour T.V. Picture Tubes", pp.9 and 10, Philips Electronic Components and Materials Division. There it is described that the colour purity depends on the axial position of the deflection unit and on the magnetic field of a colour purity magnet. The influence of the colour purity magnet extends over the whole display screen, but axial displacement of the deflection unit has no effect on the colour purity in the center of the display screen. Therefore the colour purity is first adjusted in the center of the display screen by means of the colour purity magnet and the colour purity over the whole screen is then adjusted by moving the deflection unit along the tube axis.
In another prior art device, a deflection unit, the magnetic core of the deflection unit consists of a number of adjustable segments. The segments can be moved with respect to each other in the axial direction or in a plane perpendicular to the display tube longitudinal axis. Small convergence errors can be removed by moving the segments. A construction with separate movable segments is very complicated and hence not suited for series production. The colour purity adjustment of this tube and deflection unit takes place in the manner already described by moving the deflection unit along the tube axis.
Since nowadays it is possible to manufacture the display tube envelopes with very small tolerances, it is possible to provide them with deflection units which are adjusted so as to be colour pure and which, when mounted on said identical display tubes, need no longer be adjusted so as to provide a colour pure display. It is also possible to secure adjusted deflection units rigidly to the display tube.